Of course the Zoo is the much better choice for ordinary zoovisitors.
First, the zoo is within the city center and most people visiting Berlin live in a hotel within the city-the journey to the Tierpark takes from the city 45 Minutes, and you have to change the U-Underground lines. The Zoo is defenitley much better to reach.
2.The Walking distances between the favorite species for ordinary visitors-elephants,rhinos,hippos,giraffes,zebras,seals,penguins,polar bears, big cats, apes and monkey, are much shorter than at The Tierpark, and the Tierpark houses to much "boring"animals...
3.The Zoo has a lot of animal houses-so it is the prefect choice for a rainy day, and the zoo has the Aquarium.
4.I may be wrong, but I could imagine, the zoos landscape with the old animal buildings in it could be more beautyful to ordinary visitors than the Tierpark-which is, of course, also beautyful, but diffrent.